Typical health regulations require that dishwashers in public establishments be supplied with rinse water of at least 180.degree. F. for purposes of sterilization. Water at this temperature is too hot for other hot water uses, such as the washing of hands, and is unnecessarily hot for other purposes such as the washing of clothing. In such establishments, water at two temperatures must be available.
The prior art has provided several ways in which this can be done. One common scheme is to provide a hot water heater which heats and stores hot water at 180.degree. F. and a mixing valve for mixing this water with cold water as needed to satisfy a demand for water of a lower temperature. This is inefficient, because the heat lost in storage of water at higher temperatures is greater over time than from water stored at lower temperatures. In particular, because dishwasher usage is typically intermittent, storage of a large quantity of water at 180.degree. F. is wasteful of energy. Those skilled in the art will also be aware that unnecessarily high water temperatures are harder on equipment, and require more maintenance thereof, as the amount of mineral precipitates increases exponentially with temperature.
Heeger U.S. Pat. No. 3,007,470 shows another system in which the improvement is that separate 180.degree. F. and lower temperature (typically 140.degree. F.) water storage tanks are provided. This is unduly complex. Other references show systems in which two thermostats are provided. The selection of one is made by an operator when it is anticipated that water at a higher temperature will be required for rinsing dishes after being washed. See, e.g., Swindle U.S. Pat. No. 4,467,178 and Pankow U.S. Pat. No. 2,480,302. See also Lowe U.S. Pat. No. 3,351,130.
All of the latter systems referred to above have the common defect of requiring operator intervention. This raises the possibility of error on the part of the operator. This is a serious matter, especially as the public health is at issue in connection with the sanitation of dishes. Therefore, it would be desirable to provide an automatic system for ensuring that water of adequate heat to ensure sterility of the dishes is always provided whenever a commercial dishwasher is operated, so as to provide foolproof sterilization.
To provide such a system is therefore an object of the invention.
A further object of the invention is to provide a hot water heating system in which hot water is stored at a temperature less than the temperature required for sterilization, e.g. at 140.degree. F., when sterilization of dishes is not required, but in which the water is automatically heated to a temperature high enough to achieve sterilization whenever the dishwasher is run. In this way a foolproof, highly reliable dishwashing system in which sterilization is automatic is provided.